implementing runnable interface is better than extending to
thread why because if u implements from runnable interface u
can have chance to extend one more class.but incase of
extending thread this facility is not there

The Runnable technique is particularly convenient when you 
want to create just a single thread to carry out a specific 
task. The run() method will have access to the instance 
variables in the Runnable object. For example, a common 
approach to applet animation is to make the applet 
Runnable. The run() method then has access to the applet's 
variables, which could be parameters passed in the applet 
tags or set by the user via the graphical interface. 

Note also that since there are no restrictions on the 
number of interfaces that a class can implement, it is easy 
to add Runnable to a class.

On the other hand, if you want to create multiple threads, 
then it usually makes more sense to use a Thread subclass. 
This helps to better conceptualize the threads as 
independent objects, each with its own independent sets of 
variables (as opposed to sharing variables in the Runnable 
object.) You can set the values of whatever parameters they 
need via their constructors or "setter" methods.